Conductive polymer compositions and electrical devices comprising them are well-known. Reference may be made, for example, to U.S. Pat. Nos. 4,177,376, 4,459,473, 4,719,335, 4,722,758, and 4,761,541, and copending, commonly assigned application Ser. Nos. now abandoned, 53,610, filed May 20, 1987 (Batliwalla, et al.) now U.S. Pat. No. 4,777,351, 75,929 filed Jul. 21, 1987 (Barma, et al.), 89,093 filed Aug. 25, 1987 (Kleiner et al.), 189,938 (Friel filed May 3, 1988, 202,165, (Oswal, et al.) filed Jun. 3, 1988, 202,762 (Sherman, et al.) filed Jun. 3, 1988, 247,059 (MP1271, Shafe et al.) filed Sep. 20, 1988, 247,026 (MP1272, Shafe et al.) filed Sep. 20, 1988, 252,237 (MP1247, Newman) filed contemporaneously with this application, International Application Ser. No. PCT/US88/02484 (Barma et al) filed Jul. 21, 1988, the disclosures of which are incorporated herein by reference.
The resistivity of the conductive polymer composition comprising an electrical device is important in determining the operating characteristics of the device, e.g. the resistance of the device and the power density when connected to a suitable source of electrical power. The resistivity is primarily a function of the content and resistivity of the conductive filler which is incorporated into the conductive polymer composition, but it may be affected by processing conditions, e.g. the method of compounding, type of additives, or level of irradiation. When an article is prepared from the conductive polymer composition by melt-forming (e.g. extrusion or injection molding), the conditions of the melt-forming process may be an important factor in determining the resistivity of the article and the resulting electrical performance.
It is known that the resistance uniformity of a strip of melt-extruded conductive polymer is greater in the direction of extrusion of the strip (i.e. the "machine" direction) than it is in either of the perpendicular (i.e. the "transverse" and "normal") directions. In conventional conductive polymer heaters, the principal direction of current flow through the conductive polymer is in the transverse direction (for strip heaters) or the normal direction (for laminar heaters). Because resistance uniformity results in improved power output, voltage stability, and thermal profile, it is desirable that the predominant direction of current flow be in the direction of greatest resistance uniformity. This may be particularly difficult to achieve for heaters in which the resistivity of the conductive polymer composition is very high. For such materials, small variations in filler concentration often result in large differences in resistivity and corresponding large variations in resistance uniformity. For laminar sheet heaters, traditional extrusion dies have produced extruded sheet in which the orientation is inherently nonuniform due to the unequal residence time of the polymer melt at the center and edges of the sheet. Such dies have required a plurality of cartridge heaters inserted at the lip of the die. Non-uniform heating of these cartridges has resulted in hot- and cold-spots in the die, producing non-uniform resistances. In addition, differential shear and non-uniform melt-viscosity across the die have contributed to non-uniform resistances. It is common to cut the edges off the extruded sheet produced with "coat hanger" design dies in order to eliminate the resistance variations at the edges due to shear and inconsistent flow.
Various heater geometries have been proposed to maximize the current flow in the direction of highest resistance uniformity. U.S. Pat. No. 4,459,473 (Kamath) discloses a strip heater in which an elongate resistive heating strip is in electrical contact alternately with a first and then a second spaced-apart elongate conductor. In a preferred embodiment, the conductive polymer heating strip is helically wrapped around the conductors and the current flows the length of the heating strip in the direction of extrusion. U.S. Pat. Nos. 4,719,335 and 4,761,541 (both Batliwalla et al.) and copending commonly assigned application Ser. No. 53,610 (Batliwalla et al.) now U.S. Pat. No. 4,777,351 disclose laminar heaters comprising interdigitated electrodes. In these heaters, the current flows parallel to the surface of the laminar conductive polymer element, preferably in the machine direction to optimize stability.